 At the end of the 18th century, this began to be replaced by puddling (in a puddling furnace), which was in turn gradually superseded by the production of mild steel by the Bessemer process.The term refining is used in a narrower context.Henry Cort's original puddling process only worked where the raw material was white cast iron, rather than the grey pig iron that was the usual raw material for finery forges.